This invention relates to single stage, controllable fuel/air ratio combustors having fuel/air premixers for gas turbine engines. More particularly, this invention relates to combustors of the type having premixers with a bypass air valve system for affecting pressure and cooling conditions during idle or low load operations.
It is known from U.S. Pat. No. 5,377,483, for example, to utilize fuel/air premixers to provide a closely controlled fuel/air mixture in single stage combustors for gas turbine engines. The close control afforded by such combustor system premixers results in good mixing, controlled flame temperatures and low levels of NO.sub.x, CO, and unburnt hydrocarbon emissions, as well as reduced losses due to the overall control of the compressed air inventory.
It is also known from the aforementioned patent that the close control of the fuel/air ratio can be advantageously achieved using a rotatable-type air valve located in the premixer to control the combustion air flow. In the referenced construction, the compressed air flow not used for combustion is used to convectively cool the combustion chamber liner before passing through dilution ports upstream of the turbine to be mixed with the combustion gases. This arrangement provides efficient use of the compressed air without disturbing the fuel/air ratio in the combustion zone.
However, for gas turbine applications requiring relatively high pressure ratios and turbine inlet temperatures, such as certain twin spool designs, the cooling air flow at idle load condition can be significantly greater than at full load, possibly leading to unacceptably high pressure drops or even choked flow conditions at the dilution ports or coolant passages. Such conditions, if they occur, can adversely affect compressor and overall cycle performance and, consequently, degrade operation and performance of the gas turbine. A cost effective solution, preferably one requiring a minimum of moving parts, is therefore needed to overcome or mitigate this potential problem.